1. Field of the Invention
The present invention relates to an optical amplifier capable of efficiently amplifying signal light of 1.3 .mu.m band.
An optical amplifier directly amplifying an optical signal as it is, without converting the optical signal into an electrical signal, is substantially bit rate-free and has such features that it facilitates construction of a large-capacity system and enables multiple channels to be amplified en bloc. Because of such features, intensive studies are being made in various research institutions on the optical amplifier as one of the key devices in the optical communication system for future. As one of the types of such optical amplifier, there is an optical fiber amplifier using a doped fiber, an optical fiber chiefly the core of which is doped with a rare earth element such as Er (erbium). The optical fiber amplifier has such excellent characteristics that it provides high gain, the gain is not dependent on polarization, it produces low noise, and it incurs little connection loss at its connection with an optical fiber as a transmission line. Hence, research on the development of an arrangement suitable for the wavelength of signal light to be amplified is being made in various fields.
1. Description of the Related Art
As an optical fiber amplifier for amplifying signal light of 1.5 .mu.m wavelength band, one using Er as the doping element is well known. This optical fiber amplifier has already reached a status as practicable device exhibiting low noise and highly efficient optical amplification. On the other hand, as an optical fiber amplifier for amplifying signal light of 1.3 .mu.m wavelength band, one using Nd (neodymium) as the doping element is being under development. However, since the fluorescence peak of Nd has a slightly longer wavelength than the 1.3 .mu.m wavelength band, there is a problem with the optical fiber amplifier using Nd as the doping element that a sufficient gain cannot be obtained. Further, it emits light in the vicinity of the wavelength 1.0 .mu.m and this adversely affects the stimulated emission of the 1.3 .mu.m wavelength band. Although there have been made such proposals as to shift the wavelength of the fluorescence peak toward the lower wavelength side by changing the composition of the glass forming the doped fiber or to remove the fluorescent light of 1.0 .mu.m wavelength band by inserting an optical coupler in the device, it is the present state that only a gain around 10 dB can be obtained from it.